Method of operating and amperometric measuring cell

ABSTRACT

The invention relates to a method for operating an amperometric measuring cell which includes at least a measuring electrode 2 and a counter electrode 3 in an electrolyte chamber 4 filled with an electrolyte. The measuring cell is closed off by a permeable membrane 7 with respect to the measurement sample to be detected. The method of the invention improves the run-in performance of the measuring cell 1. The method includes the step of applying a voltage U 1  across the electrodes (2, 3) during a first time span T 1  starting at a reference time T 0 . A reference voltage U 0  is assumed at the start of the measurement and the voltage U 1  is increased relative to the reference voltage U 0 .

FIELD OF THE INVENTION

The invention relates to a method for operating an amperometric measuring cell which includes at least a measuring electrode and a counter electrode in an electrolyte chamber which is closed by a permeable membrane with respect to the measurement sample to be detected. The measuring cell is connected to a voltage source supplying a voltage and generating a sensor current between the electrodes.

An electrochemical measuring cell of the above kind is disclosed in U.S. Pat. No. 4,961,834 incorporated herein by reference. In this measuring cell, a measuring electrode, a reference electrode and a counter electrode are arranged in an electrolyte chamber of the measuring cell housing. The electrolyte chamber is filled with an electrolyte and the housing is closed off by a permeable membrane with respect to the measurement sample to be detected. The measuring electrode, the reference electrode and the counter electrode have respective connecting Leads which pass through the measuring cell housing and are connected to an evaluation unit having a voltage source. After the voltage source is switched on, a specific sensor current flows which drops to a steady-state end value after a certain time. This end value can be referred to as the sensor rest current.

It is a disadvantage of this known measuring cell that the sensor rest current is only reached after a longer time and therefore affects the evaluation of the measuring signal by the sensor current which changes continuously and which approaches the sensor rest current asymptotically.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for improving the run-in performance of an electrochemical measuring cell.

The method of the invention is for operating an amperometric measuring cell for measuring a sample. The measuring cell includes: an electrolyte chamber having an opening directed toward the sample to be measured and holding an electrolyte; a permeable membrane mounted on the chamber for closing off the chamber; and, a measuring electrode and a counter electrode disposed in the chamber so as to be in spaced relationship to each other. The method includes the steps of: providing a voltage source outputting a voltage U for applying the voltage U across the electrodes to generate a sensor current i(t) between the electrodes; and, starting with the voltage U across the electrodes at a reference voltage U₀ at a reference time T_(O) and applying a voltage U₁ during a first time span T₁ measured from the reference time T_(O) with the voltage U₁ being increased relative to the reference voltage U₀.

The advantage of the invention is seen essentially in that a significantly shortened run-in time is provided for the electrochemical measuring cell by altering the voltage on the electrodes of the measuring cell to a first voltage U₁ during a first time span T₁. This first voltage U₁ is increased compared to the reference voltage U₀. The improvement is produced in that the capacitors, which are defined by the electrodes of the measuring cell with the electrolyte disposed therebetween, are charged to the reference voltage U₁ more rapidly by the changed voltage so that the sensor current i(t) adjusts more rapidly to the sensor rest current i₀. The electrolyte can be in a solid form as a solid-state electrolyte. The electrolyte can also be in liquid form or be a gel.

An advantageous embodiment of the invention comprises applying a second voltage U₂ to the electrodes during a second time span T₂ which follows the, first time span T₁ with the second voltage U₂ reduced with respect to the reference voltage U₀. The time spans T₁ and T₂ are selected so that the second time span T₂ is not greater than the first time span T₁. A further shortening of the run-in time of the measuring cell is obtained by dropping the voltage U below the reference voltage U₀ during the second time span T₂.

The voltages U₁ and U₂ and the time spans T₁ and T₂ are so selected that the product of the amount of the difference U₁ minus U₀ and T₁ divided by the product of the amount of the difference U₂ minus U₀ and T₂ is equal to or greater than five as expressed below: ##EQU1##

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings wherein:

FIG. 1 is a schematic of an amperometric measuring cell having two electrodes;

FIG. 2a shows the sensor current i(t) plotted as a function of time when a reference voltage U₀ is applied with the latter being constant on a graph of voltage U(t) versus time (t);

FIG. 2b shows the graph of voltage U(t) versus time (t) corresponding to the plot of sensor current shown in FIG. 2a;

FIG. 3a shows the sensor current i(t) plotted as a function of time when a voltage U₁ is applied which is increased with respect to the reference voltage U₀ during time span T₁ and reduced to voltage U₂ in time span T₂ (begin new paragraph) FIG. 3b shows a graph of the voltage U(t) plotted as a function of time (t) corresponding to the plot of sensor current shown in FIG. 3a;

FIG. 4 shows an equivalent circuit of the measuring cell of FIG. 1; and,

FIG. 5 shows an equivalent circuit of an electrochemical measuring cell having an additional reference electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a schematic configuration of an electrochemical measuring cell 1 having a measuring electrode 2 and a counter electrode 3. The electrodes (2, 3) are arranged in an electrolyte chamber 4 of a housing 5 of the measuring cell 1. The measuring cell housing 5 is filled with an electrolyte 6 in the form of an aqueous solution and is closed off with respect to the gas sample to be detected by a permeable membrane 7. The electrodes (2, 3) are connected via lines (8, 9) to a voltage source 10. A voltage U is applied across the electrodes (2, 3) by means of the voltage source 10. The sensor current i(t) is tapped off as a voltage drop across a measurement resistor 11 in the line 9.

FIGS. 2a and 2b show the sensor current i(t) as a function of time when a constant reference voltage U₀ is applied across the electrodes (2, 3) at time point t=T₀. The time T_(O) is coincident with the origin of the coordinates. The sensor current i(t) first increases to a maximum value i_(m) and then drops to the sensor rest current i₀. The time span until the sensor rest current i₀ is reached is dependent upon the construction of the measuring cell 1 and can be up to 24 hours. In this way, a continuously changing sensor current i(t) must be accepted during this time interval after the measuring cell 1 is taken into service. This changing sensor current i(t) affects the evaluation of a concentration measurement.

FIGS. 3a and 3b show the run-in performance of the measuring cell 1 operated pursuant to the method of the invention. During a first time span T₁, a voltage U₁ is applied to the electrodes (2, 3) with this voltage U₁ being increased with respect to the reference voltage U₀. A second voltage U₂ is switched on to the electrodes (2, 3) during a second time span T₂ and this voltage U₂ is less than the reference voltage U₀.

In the present case, the first voltage U₁ corresponds to the value of the reference voltage U₀ multiplied by a factor of 1.5 and the first time span T₁ amounts to approximately 1 hour. The second voltage U₂ is adjusted to half the value of the reference voltage U₀ and the second time span T₂ is approximately 12 minutes.

The sensor current i(t) reaches the sensor rest current i₀ already after approximately one hour and 12 minutes as shown in FIG. 3a. This compares favorably to the 24 hours which are needed utilizing the state of the art as shown in FIGS. 2a and 2b.

FIG. 4 shows an equivalent circuit of the measuring cell 1 of FIG. 1 and the method of the invention will now be explained with respect to FIG. 4. The elements of the measuring cell 1 are here represented as electrically equivalent resistors and capacitors. R₁ is an input and contact resistor of the measuring electrode 2 and C₁ is a capacitor defined by the electrodes (2 and 3) and the electrolyte 6. The time constants at which the Faraday current in the measuring cell 1 decays can be represented by a series circuit of a resistor R₂ and a capacitor C₂ in the equivalent circuit.

The capacitor C₂ charges slower than the capacitor C₁ when a voltage U is applied across the lines (8, 9). This is so because of the series circuit of the resistors R₁ and R₂.

Applying the first potential U₁ across the lines (8, 9) of the measuring cell 1 causes the charging of the capacitor C₂ to be accelerated. It is especially advantageous when the capacitor C₂ reaches the reference voltage U₀ after the first time span T₁ has elapsed. The application of the second voltage U₂ during the second time span T₂ causes the voltage on the capacitor C₁ to again drop to the reference voltage U₀. The voltage of the capacitor C₁ had a higher value than the reference voltage U₀ during the first time span T₁. After the time spans T₁ and T₂ have elapsed, the capacitors C₁ and C₂ are charged to the reference voltage U₀ and the sensor current i(t) assumes its sensor rest current i₀ directly after the end of the second time span T₂.

The voltages U₁ and U₂ as well as the time spans T₁ and T₂ are, in this present case, so selected that the product of the amount of the difference U₁ minus U₂ and T₁ divided by the product of the amount U₂ minus U₀ and T₂ is equal to five as follows: ##EQU2##

The method of the invention is applicable in the same manner to a three-electrode measuring cell 12 having a reference electrode. The equivalent circuit of such a measuring cell is shown in FIG. 5. The same components of FIG. 5 are shown with the same reference numerals as in FIGS. 1 and 4. The reference electrode (not shown in FIG. 5) is connected to a line 13 of the three-electrode measuring cell 12.

It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A method of operating an amperometric measuring cell for measuring a sample, the measuring cell including: an electrolyte chamber having an opening directed toward the sample to be measured and holding an electrolyte; a permeable membrane mounted on said chamber for closing off said chamber; and, a measuring electrode and a counter electrode disposed in said chamber so as to be in spaced relationship to each other; and, the method comprising the steps of:providing a voltage source outputting a voltage U for applying said voltage U across said electrodes to generate a sensor current i(t) between said electrodes; and, starting with said voltage U across said electrodes at a reference voltage U₀ at a reference time T_(O) and applying a voltage U₁ during a first time span T₁ measured from said reference time T_(O) with said voltage U₁ being increased relative to said reference voltage U₀, wherein the sensor current i(t) adjusts more rapidly to a sensor rest current i₀ to shorten the run time of the cell.
 2. The method of claim 1, further comprising the step of applying a second voltage U₂ across said electrodes during a second time span T₂ directly after said first time span T₁ with said second voltage U₂ being dropped relative to said reference voltage U₀ ; and, selecting said first and second time spans (T₁, T₂) so that said second time span T₂ is not greater than said first time span T₁.
 3. The method of claim 2, wherein said voltages (U₁ and U₂) and said time spans (T₁ and T₂) are so selected that the product of the difference (U₁ -U₀) and T₁ divided by the product of the difference (U₂ -U₀) and T₂ is equal to or greater than 5 as follows: ##EQU3## . 